(1) Field of the Invention
The present invention relates to a process of and an apparatus for preparing a laminated strip conprising a pair of heat-shrinkable sheets fused together with a heating element, such as an electric conductor wire to which electricity is to be applied, therebetween.
(2) Description of the Prior Art
Such a laminated heat-shrinkable strip with wire embedded therein is well known as a means for covering and protecting joints of steel pipes of oil pipelines or cables. The strip is formed by fusing two heat-shrinkable sheets of, for example, polyethylene, placed one on top of the other with a wire of, for example, copper, therebetween. The sheets have the property of being thermally shrinkable in the length wise direction. The wire should not be aligned parallel to the shrinking direction but traversely thereto at an approximately right angle, thus has preferably been arranged in a zig-zag manner as shown in FIG. 1, in which numerals 1 and 2 denote the strip and the wire, respectively. The finished strip used as sealing means has a predetermined length and a pair of electric terminals to be connected to a power source. The terminals are on the two ends of the wire exposed from the strip at the opposite ends of the strip. The heat-shrinkable strip is used in the following manner. The strip is wound around a steel pipe or cable to cover the joint portion entirely. Electricity is then applied through the terminals of the wire embedded in the strip to heat the wire. The zig-zag arranged hot wire heats the entire strip to shrink it so that is is in a tight contact with the joint portion. This tight contact protects the joint from surface corrosion and improves sealing of the pipe at the joint. Such effects are ensured for a long period, even though they rely on the durability of the material forming the heat-shrinkable sheet.
In the conventional process, heat-shrinkable strips are prepared as shown in FIG. 3. Referring to FIG. 3, a pair of upper and lower sheets of polyethylene 3 and 3' and a copper wire sandwiched, in the zig-zag form shown in FIG. 1, therebetween are forced to pass through nibs of plural pairs of upper and lower heating or hot rolls 4 and 4'. These pairs of hot rolls 4 and 4' are positioned in a zig-zag arrangement as seen in cross-section, as shown in FIG. 3. The pair of the upper sheet 3 and lower sheet 3' are fused and press-fitted to each other while passing through each pair of rolls 4 and 4' by the pressures exerted by the rolls. Since the upper sheets 3 and the lower sheet 3', placed one on top of the other with uniformly zig-zag arranged wire 2 sandwiched therebetween, as shown in FIG. 1, are forced to run forward and up and down in a zig-zag manner while being pressed and fused by the rolls 4 and 4', the rolls are likely to cause the upper and lower sheets 3 and 3' to bulge from each other at portions 3a and 3'a, indicated by the lines dotted just before contact with the rolls. In so bulding, the press-fitted sheets become partially disengaged. When the bulged portions are passed though the rolls, the result is wrinkles. Further, repeated bulging allows undesirable entrance of air between the upper and lower sheets. Air in the laminated strip thermally expands and thus further enlarges the bulged portions while the strip is running through the series of rolls.
It is preferred that the copper wire 2 remain as originally arranged, that is, as shown in FIG. 1, when the laminated strip 1 is completed. However, since, according to the conventional process, the polyethylene sheets 3 and 3' with the wire 2 sandwiched therebetween are fused by means of a series of the hot rolls 4 and 4' while the sheets are forced to move forwardly continuously by the hot rolls, the uniformed zig-zag arrangement of the wire 2 between the sheets tends to be distorted as shown in FIG. 2. In the distorted arrangement, the gap between the neighboring portions of the wire 2 varies, resulting in a nonuniform density of the wire per unit area of the strip. Such a nonuniform distribution of wire density leads to nonuniform heating of the strip over the entire surface area thereof when electricity is applied to the wire. That is, heat energy generated per unit area by the wire varies at different areas of the strip. This results in a different amount of heat-shrinkage at different areas of the strip. In such a case, the strip does not completely cover the surface of the joint of the steel pipe or cable, that is, gaps appear between the strip and the pipe or cable surface, thereby not achieving or ensuring complete protection of the joint against corrosion.